Employees working split shifts could be harming their health, Health and Safety Executive research has suggested.
A study found offshore oil rig workers who worked seven nights then seven days had a higher risk of heart disease than those who stayed on days or nights.
Split-shift workers were also more tired and inattentive, New Scientist magazine reported.
Occupational health expert Cary Cooper said involving staff in designing shift patterns reduced health problems.
Researchers from Cardiff University and the University of Guildford in Surrey examined the health of 45 offshore oil rig workers.
Both compared the effect of the two main shift schedules on the men.
Over two weeks, they may have been asked to work 12 hour day-shifts or night-shifts.
But the other group worked a split-shift of seven night-shifts followed by seven day-shifts. Many preferred this pattern because it got them into the habit of night-time sleeping before they went home.
However, the researchers found it was worse for their health.
Urine tests from men working the split-shift pattern showed levels of melatonin, a hormone which regulates sleep and which is normally secreted at night, did not become synchronised to new sleep times after shift changes.
The men also had higher levels of fatty acids circulating in their blood after meals, compared with the day-shift or adapted workers, indicating they were at higher risk of heart disease, diabetes and other metabolic disorders. 'Wider implications'
Josephine Arendt, a chronobiologist from the University of Surrey, who carried out one of the studies, told New Scientist magazine: "The swing shift is the killer."
Andrew Smith of Cardiff University, who carried out the second study, said workers should try to avoid split-shifts and other schedule changes that put their body clocks out of kilter.
He said legislation which forced companies to stick to particular schedules was unlikely, but he said people who did work split-shifts could minimise the risks to their health by avoiding fatty and sugary snacks at night.
A spokesman for the HSE told the BBC News website it had invested �1m into a raft of studies into the effect of shift patterns on offshore oil-rig workers.
He added: "The offshore oil rigs represent an excellent natural laboratory with controlled conditions for work, rest, sleep and meals and it is already becoming apparent that this research will have implications in many other sectors of employment.
"When the programme is over, HSE intends to review the evidence and consider appropriate action, which could include producing guidance for both the industry and its own Inspectors on how fatigue and shift work should be managed and optimised for health, safety and performance."
Professor Cary Cooper, based at Lancaster University, said: "If you look at stress and health, people who feel they have some control over their working lives are then less likely to have illnesses." BBC News